It has previously been proposed in U.S. Pat. No. 4,194,324 of Bornora et al. issued Mar. 25, 1980 to provide a semiconductor wafer polishing machine including a wafer carrier in the form of a solid plate having a load bearing flange of circular configuration supported on its upper surface by a plurality of webs which extend inwardly and outwardly of the flange to increase the strength of the carrier and to dissipate the heat produced in the carrier during polishing. The carrier is coupled to a rotating head by vacuum pressure and the head is connected by a spherical ball and socket type connector to a rotating drive shaft. The rotating drive shaft is hollow and is provided with a vacuum passage within which a spring biased valve is mounted for controlling the air pressure applied to the interface between the carrier and the rotating head in response to the polishing force to release such vacuum pressure during polishing. The polishing machine of the present invention does not employ these features and is capable of more accurate polishing of the flat surface of the wafer. Thus the wafer polishing machine of the present invention employs a wafer carrier with a rigid foam core for low mass and high strength. It also employs a flexible kinematic coupler to couple the rotation of a motor driven shaft to the wafer carrier through a load transfer plate and employs a separate adjustable load pressure device for applying polishing pressure to the transfer plate through an air thrust bearing which isolates the load pressure device from the rotational force.
A wafer polishing machine is shown in U.S. Pat. No. 5,443,416 of Volodarsky et al. issued Aug. 22, 1995 that includes a wafer carrier coupled to a rotary drive shaft by a flexible diaphragm which is in the form of a flexible disk like membrane that also serves as a pressure seal for a load pressure chamber within the rotating polishing head which applies a load pressure to the carrier during polishing through the diaphragm by means of an air source which is coupled through a rotary coupling to such chamber during polishing. This wafer polishing machine has a perforated carrier plate with a region having passages there through from the top surface to the bottom surface and a cushioning pad in communication with the wafer held on such bottom surface to enable easier removal of the wafer after polishing. The carrier is mechanically tilted relative to the polishing table on which the polishing grit is supported by a felt layer to enable easier removal of the wafers from such felt after polishing. In addition, a retaining ring is provided around the carrier to provide a pocket for insertion of the wafer so that it is in alignment with the carrier. Also, a lifting shelf and lifting prongs are employed for mechanically tilting of the carrier during removal of the wafer. These features result in an extremely complicated wafer polishing machine which is difficult to operate satisfactorily and are not employed in the polishing machine of the present invention. In addition, because the shaft rotation force for rotating the carrier and the load pressure force for pressing the carrier towards the polishing table are both transmitted through the same flexible diaphragm in such patent there is no way to separate these forces and independently adjust them as is true of the present invention. Thus in the polishing machine of the present invention the load force for urging the carrier toward the polishing table is applied to a load transfer plate through a bearing separately from the rotational force applied by the flexible coupling. The load force is produced by a pneumatic bellows or other equivalent device and is transmitted through an air lubricated thrust bearing to the load transfer plate to reduce friction and to isolate the load force pressure device from the rotation coupling element.
U.S. Pat. No. 5,205,082 of Shendon et al. issued Apr. 27, 1993 shows a wafer polishing machine which is similar to that of U.S Pat. No. 5,443,416 in that it employs a rotatable polishing head connected to a wafer carrier by a flexible diaphragm that also functions as a membrane seal for an air pressure chamber which applies a load pressure to the carrier through the diaphragm to urge the wafer against the polishing surface. Thus the flexible diaphragm functions not only as a flexible coupling for applying the rotational force to the carrier but also functions to apply the load pressure force to the carrier during polishing. U.S. Pat. No. 5,081,795 of Tanaka et al. issued Jan. 21, 1992, shows a similar teaching. In addition, the polishing head employs a retainer ring around the carrier to provide a pocket for receiving the wafer on the carrier. Such polishing head employs positive pressure and a movable stop to extend the carrier downward beyond the retaining ring to enable the wafer to be removed from the carrier after polishing. In both of these prior polishing machines the load pressure transmitting diaphragm which also serves as the flexible coupling for coupling the carrier to the rotating head, rotates and moves vertically in the direction of the Z axis due to load pressure changes and variation in the thickness of the wafer during polishing. This vertical and rotational movement of the diaphragm causes corresponding movement in the axis of rotation of the carrier which results in inaccurate polishing of the wafer so that it is not polished as flat as it otherwise could be. The polishing machine of the present invention overcomes this problem by not applying the load pressure to the carrier through the flexible rotation coupling but instead applying the load pressure through an air thrust bearing to isolate it from the rotational source.
Another cause of inaccuracy in polishing is the irregular movement and vibration of the carrier due to the high mass of the solid metal carrier used for the rotating polishing head in the above patents which produces an undesirable effect on polishing accuracy. The polishing machine of the present invention avoids this problem by employing rigid foam core elements for the carrier and the load transfer plate which reduces their mass and increases their resonant vibration frequency for easier dampening. The air bearing in the polishing machine of the present invention permits greater freedom of tilting for the carrier and its load transfer plate. Also, the air bearing isolates the load pressure source from the rotating force of the spindle which is coupled to the load transfer plate by a flexible coupling. The tilting of the load transfer plate relative to the load pressure source in the present invention is enabled by the air bearing which slides relative to the load transfer plate. In addition, the air bearing provides a dampening effect on any resonant vibration of the carrier or the load transfer plate which might otherwise be transmitted across the air film of such bearing thereby reducing vertical axis movement of the carrier during polishing and producing a flatter surface finish on the wafers.
It has also been proposed in U.S. Pat. No. 4,918,870 of Torbert et al. issued Apr. 24, 1990 to provide a wafer polishing machine in which a plurality of floating sub-carriers are mounted on a conventional wafer carrier in an attempt to provide low cost polishing wafers. However, the polishing machine of this patent employs a mechanical spring or pneumatic/hydraulic devices for axial loading of the sub-carrier. By failing to isolate the polishing load force on the sub-carrier from the rotational device for rotating the sub-carrier the polishing machine of Torbert does not produce highly accurate flat polishing of the wafers.
U.S. Pat. No. 3,603,042 of Boettcher issued Sep. 7, 1971 and U.S. Pat. No. 3,731,435 of Boettcher et al. issued May 8, 1973 show polishing machines in which the carrier is coupled to a rotating shaft by a vacuum and the carrier is cooled by fluid flowing through the carrier to dissipate the heat produced during polishing. Also these patents show the use of valve means for changing the pressure applied to the head to retain the wafer or the work piece on the head and to release it once polishing is finished.